Epoxy resins are widely used in industry for numerous applications including coatings and laminating adhesives. Manufacturers are not content simply to adapt their processes to the epoxy products available but insist that such resins be tailored to meet their needs. To remain competitive it is essential that suppliers of epoxy resins continually improve their products with their customers' requirements in mind.
One approach for modifying epoxy polymer products has been by combining the epoxy resin with other polymers which provide certain advantages in their physical properties and processibility. This is complicated by another objective which is to place the combined polymers in aqueous dispersion for handling and use in order to avoid toxicity and fire hazards associated with organic sol vents. Such combinations often have three serious disadvantages. One is incompatibility of the combined polymers, resulting in the final products having inferior mechanical strength and poor appearance. A second problem is instability of emulsions of the combined polymers owing to incompatibility of emulsions of each of the component polymers. The third difficulty frequently encountered is inadequate curing of the hybrid polymer because of the surfactant or emulsifier layer on the polymer particles of each constituent. In the past these problems have been addressed in different ways involving various polymerization recipes and blending sequences.
Evans, et al., U.S. Pat. No. 4,212,781 (1980) describes a method of modifying epoxy resins by graft polymerization. The addition monomer, such as an acrylate system, is polymerized by free radical initiation in the presence of the epoxy resin, either in the absence of a solvent or, preferably, in a two part system of miscible solvents, one of which dissolves the epoxy resin and the other the addition monomer, for example, benzene and ethanol. The product is said to be a blend of graft polymer, ungrafted polymerized addition polymer and unreacted epoxy resin. This solution of mixed polymers can be converted to an aqueous system by replacing the solvent 10 with water using a base such as dimethylethanol amine. This procedure, although complex, is said to produce a stable emulsion. Epoxy/acrylate hybrid polymers prepared by this process, however, have very limited epoxy groups left and are typically cured through the hydroxyl groups with either aminoplast resins or urea formaldehyde resins at high temperature.
Hybrid polymer products are sometimes made simply by blending different polymers together. Merz, et al., U.S. Pat. No. 4,377,433 (1983) describes a blended aqueous dispersion of (1) an epoxy compound, (2) a copolymer of a vinyl or acrylic acid ester and an ethylenically unsaturated monomer having hydroxy or carbonyl groups, and (3) an amine hardener. Stability (pot life) of this dispersion is said to be only from a few hours to about 2 weeks.
In an effort to solve this stability problem, Chu, et al., U.S. Pat. No. 4,446,258 (1984) modified one polymer so it would serve as a dispersant for the other. A can coating composition is described which is an aqueous dispersion of (1) a neutralized reaction product of an epoxy resin with an acid polymer, such as an acrylic prepolymer, and (2) a second resin, not necessarily self-dispersible in water, either preformed or prepared in the presence of (1). Stability of this mixture is said to be provided by the epoxy compound which acts as a polymeric surfactant. The coating composition can be cured with an agent such as an aminoplast or a phenoplast resin.
Minnis, et al., U.S. Pat. No. 4,588,757 (1986) discloses a two-part laminating adhesive which includes as one part an emulsion made by polymerization of ethylenically unsaturated monomers, such as an acrylate monomer system, in the presence of an epoxy resin. A polyfunctional amine is added as the second part at the time of use. It is said that the emulsion can be stored for an indefinite period of time before adding the amine. The object is to avoid use of solvents in the polymerization. The polyepoxides are emulsified in water using nonionic surfactants, although the surfactant used is said to be of secondary significance. Such surfactant can be an ethylene oxide adduct of octyl or nonyl phenols or long chain alcohols such as dodecanol. The acrylic monomers are also emulsified with a nonionic surfactant alone or with an anionic surfactant like sodium lauryl sulfate. These are water soluble emulsifiers commonly used in emulsion polymerization recipes. In the sole example, a preemulsion mixture of water, surfactant, epoxy compound and acrylate/acrylic acid monomer was added slowly to hot water containing persulfate initiator. Little grafting is said to be expected. This product, when blended with polyamine emulsion, is said to be a relatively stable mixture, but having a pot life of only about one week.
In a more recent approach to this problem, Davis, U.S. Pat. No. 5,037,700 (1991) discloses a laminating adhesive consisting of (1) an emulsified copolymer of acrylate, styrene or vinyl ester and an ethylenically unsaturated carboxylic acid, (2) an epoxy resin emulsion and (3) a polyfunctional amine. The copolymer (1) is prepared independently by aqueous emulsion polymerization using a suitable dispersant and then mixed with epoxy resin solids in aqueous emulsion. It is said that this mixture can be stored indefinitely, but the amine is added only at the time of using the adhesive to make a laminate.
It is highly desirable to find a way to make an aqueous emulsion of a hybrid epoxy resin which can be used in coating applications to provide a film having good clarity, water and solvent resistance, and adhesion to a number of different substrates. Preferably, such an emulsion should have stability over an extended period of time and be useful in a one-pack system which does not require the metering of two reactive components at the time of use. Also, the polymers should be curable at relatively low temperatures.
The concept of using various mixtures of surfactants, both oil soluble and water soluble surfactants, has been known for many years in the microemulsion field of technology and widely applied in the cosmetic, pharmaceutical and food industries. There is no known application of this concept, however, in emulsion or dispersion polymerization for the modification of epoxy resins, and apparently is little known or rarely utilized in the emulsion polymerization arts.